The organic compounds undergoing a reversible change of color on exposure to photic or thermal energy which are typical and best known are spiropyran derivatives and G. H. Brown: Photochromism (John Wiley & Sons, Inc., 1971), for one, can be consulted for a specific listing of such derivatives and their physical properties.
However, when an attempt is made to exploit the hitherto-known spiropyran derivatives commercially in recording materials, they are found to have serious disadvantages. Thus, because the meta-stable species (ring opened isomers) are lacking in thermal stability in solutions as well as in polymeric binders, they tend to promptly revert to the ground state or fade out so that the chromic response at a necessary level cannot be sustained for a sufficient duration.
It is an object of the present invention to provide a compound free from the above-mentioned disadvantages of the prior art spiropyran derivatives.
It is a further object of the present invention to provide a compound exhibiting stable photochromism.